Extreme geochemical heterogeneity in Afro-Arabian Oligocenetephras: Preserving fractional crystallization and mafic recharge

processes in silicic magma chambers

Ingrid Ukstins Peate a,!, Adam J.R. Kent b, Joel A. Baker c, Martin A. Menzies d

a Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242 USAb Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR, 97331-5506 USA

c School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealandd Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, England, UK

Received 18 October 2006; accepted 6 August 2007Available online 24 August 2007

Abstract

Four Indian Ocean deep-sea tephras can be correlated to individual Oligocene on-land silicic pyroclastic units found in theAfro-Arabian flood volcanic province (Yemen and Ethiopia), providing valuable stratigraphic marker horizons. They also preserveamong the largest geochemical heterogeneities observed in individual eruption events (SiO2: 43 to 75 wt.% and 56 to 79 wt.%).The major, trace element and isotopic variations preserved in individual shards in these ash layers provide a unique series ofsnapshots of magma chamber processes that are used to elucidate the formation and petrogenetic evolution of large volume,chemically zoned silicic magmatic systems. Banded shards within individual ash layers represent up to 85% of the SiO2 variationobserved in the entire tephra on the scale of b1 mm3, clearly demonstrating that the observed compositional variations foundwithin individual layers represent individual eruptive events. The silicic magmas are compositionally related to the underlyingMain Flood Basalt phase of volcanism in Yemen, whereas the basaltic end-member found in the tephras is compositionally distinctand related to the Upper Series mafic lavas, found intercalated with the Main Silicic Series on-land ignimbrites and tuffs. Theintermediate to silicic component of the tephras was generated by extreme fractional crystallization (F=ca. 60%) of plagioclase,anothoclase, augite, magnetite and ilmenite, as observed in on-land phenocryst assemblages.© 2007 Elsevier B.V. All rights reserved.

Keywords: Afro-Arabian flood basalt; Extreme compositional heterogeneity; Bimodal volcanism; Silicic pyroclastic volcanism; Deep-sea ash layers

1. Introduction

It has long been recognized that mafic and silicicmagmas in bimodal volcanic provinces are often com-

plexly interrelated, based on the existence of maficinclusions and enclaves in silicic extrusive and intrusiveunits (e.g. Bacon, 1986; Didier and Barbarin, 1991;Wiebe, 1994). Low-density silicic magmatic systemstrap injected basaltic magmas, and the mafic componentcan experience extensive fractionation and recharge.The overlying silicic component, which may be partly toalmost completely solidified, can be remelted and re-mobilized (e.g. Fish Canyon Tuff; Bachmann and

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! Corresponding author. Tel.: +1 319 335 1824; fax: +1 319 3351821.

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Bergantz, 2004), and silicic magma may mix or minglewith co-existing basalt. Meanwhile, the basaltic under-pinnings contribute a thermal flux that maintains therhyolite magma and promotes development of compo-sitional zonation (e.g. Smith, 1979; Hildreth, 1981;Hildreth et al., 1991; Streck and Grunder, 1999;Hildreth, 2004). This co-dependent relationship appearsto be a significant contributing factor in the establish-ment and longevity of large-volume silicic systems andtheir common association with trace element-enrichedbasaltic magmas (e.g. Baker et al., 1996b; Wiebe, 1997;Streck and Grunder, 1999). Silicic magmas in thesebimodal systems can display extreme geochemicalheterogeneity, preserved as continuous compositionalvariations or as compositional “clusters”, and theheterogeneity is thought to originate from periods ofprolonged fractional crystallization and segregation offractionation products or multiple silicic magma injec-tion events (Smith and Bailey, 1966; Smith, 1979; Speraand Crisp, 1981; Fridrich and Mahood, 1987; Huyskenet al., 1994; Cambray et al., 1995; Knesel and Davidson,1997; Mills et al., 1997; Streck and Grunder, 1997;Eichelberger et al., 2000; Hannah et al., 2002; Hildreth,2004; Cathey and Nash, 2004; Christiansen, 2005, andmany others). These variations have been interpreted toeither directly reflect the compositional state of themagma chamber at the time of eruption (e.g. Wolff et al.,1990), or result from complex magma chamberevacuation dynamics during eruption (e.g. Wilson andHildreth, 1997: Bishop Tuff, CA, USA; Brown et al.,1998: Whakamaru Group, Taupo volcanic zone, NewZealand), or a combination of these. However, therecord of compositional components is often sparse orincomplete, preserved as rare mafic enclaves orinclusions within a much more volumetrically well-represented silicic system, which itself may have beenhomogenized during eruption or emplacement, furthercomplicating the process of sampling individual com-ponents (e.g. Blake and Campbell, 1986; Freundt andTait, 1986; Wolff et al., 1990; Branney and Kokelaar,1992; Wiebe, 1994; Browne et al., 2006; Bryan, 2006).As a result, the nature of these processes can often onlybe elucidated through indirect petrologic or geochemicalevidence.

Indian Ocean tephras sourced from Afro-Arabiansilicic pyroclastic eruptions associated with Oligoceneflood volcanism provide unique snapshots of complexlyzoned, large-volume silicic magma chambers, and allowexamination of magma chamber processes in a coupledbasalt/rhyolite system. These tephras are distal productsfrom large silicic pyroclastic eruptions found on theconjugate rifted margins of Afro-Arabia and likely

represent eruption events with volumes of 100's of km3

(Ukstins Peate et al., 2003, 2005). They display some ofthe most extreme compositional variations yet observedwithin individual eruptions (SiO2: ca. 43 to 75 wt.%). Acomagmatic mafic component is present in the tephrasas relatively abundant, fresh glassy shards, whichfacilitates full compositional characterization for majorand trace element concentrations and isotope ratios, andimportantly, the continuous compositional variationswithin individual tephra layers and even withinindividual banded shards yield insights into theevolution of this system.

2. Background

Oligocene Afro-Arabian bimodal flood volcanismwas erupted over a period of ca. 6 Ma, from 31 to 26 Ma(Baker et al., 1996a; Hofmann et al., 1997; UkstinsPeate et al., 2003; Riisager et al., 2005), and can bedivided into three phases of volcanic activity based onthe dominance of mafic versus silicic volcanic products:i) the Main Flood Basalt phase (including the oldestrecognized ignimbrite at 30.1 Ma), ii) the Main Silicicseries, and iii) an Upper Bimodal phase (Fig. 1; UkstinsPeate et al., 2005). The on-land record of the MainSilicic phase in Yemen preserves a sequence of sixlarge-volume silicic pyroclastic eruptions (four ignim-brites, an airfall tuff and a caldera collapse-relatedbreccia deposit) emplaced over !120 ka (!29.6 to29.48±0.13 Ma: Baker et al., 1996a; Ukstins et al.,2002; Riisager et al., 2005; Ukstins Peate et al., 2005). Afull description of the volcanic stratigraphy, pyroclastictextures and geochemical variations of the on-land unitsis given in Ukstins Peate et al. (2005).

Based on magnetostratigraphic and temporal simi-larities, Hofmann et al. (1997) postulated that distaldeposits of some of the large-volume eruptions observedin the Ethiopian plateau were preserved as a sequence offour deep-sea ash layers sampled by the Ocean DrillingProgram (ODP) Leg 115 in the Indian Ocean (ShipboardScientific Party, 1988), !2700 km to the southeast ofthe presumed eruption sites in Afro-Arabia. Ash layerswere interpreted as primary tephra deposits by theShipboard Scientific Party (1988), as any texturalevidence to indicate re-deposition by turbidites orbioturbation is lacking. As we will demonstrate,individual banded shards can show a similar composi-tional range to that seen in each ash layer as a whole,which supports the conclusion that the tephras representindividual eruption events. Touchard et al. (2003a) usedmagnetostratigraphic studies of the Leg 115 cores fromholes 709 and 711, along with nanofossil stratigraphy,

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major element concentrations and Sr and Nd isotoperatios of the tephra layers to establish a general strat-igraphic and geochemical link to Afro-Arabian floodvolcanism. They concluded that the lower-most threetephras show little compositional variation (SiO2=71.6to 72.6 wt.%), whereas the youngest tephra shows fourdistinct groups in SiO2 concentration: 69, 70.9, 73.8 and75.5 wt.%. Touchard et al. (2003a) attributed the com-positional groups observed within the youngest tephrato result from four discrete eruptions from a singlemagma chamber during advancing stages of fraction-

ation. Additional studies have examined the magneticsusceptibility and electron spin resonance of ODP Leg115 sites to quantitatively assess tephra layer thick-nesses (Ananou et al., 2003; Touchard and Rochette,2004). Touchard et al. (2003b) studied multiple drillcores from distal localities (ODP Leg 189 Site 1168: S.Tasmanian Sea; Leg 73 Site 522: S. Atlantic; Leg 23 Site220: Arabian Sea) to constrain the presence andthickness of Afro-Arabian tephras using magnetic sus-ceptibility measurements. Tephras were not visuallyobserved in all cores, but magnetic susceptibility peakswere interpreted to represent distal deposition of dustassociated with the tephras and to reflect global-scale,stratospheric transport of ash.

The basal and uppermost ash layers (annotated hereinas 5W and 4W, respectively) from Indian Ocean ODPLeg 115 core 711A were correlated (major and traceelements, Pb and Nd isotopes: Ukstins Peate et al.,2003) to two well-dated individual ignimbrite units ex-posed in the flood volcanic succession of the conjugaterifted margins in Yemen and Ethiopia. Such a marine-continental correlation precisely constrained the tempo-ral and volcanostratigraphic context of these tephras(Ukstins et al., 2002; Ukstins Peate et al., 2005). Pre-liminary morphologic and geochemical shard analysis inour previous study documented a wide variety of shardtypes (e.g., clear bubble walls and tricuspate bubblejunctions, brown transparent shards) with a significantlygreater range in major and trace element concentrationsthan was observed by Touchard et al. (2003a). Vari-ations in shard morphology reflected geochemicalheterogeneity in both major and trace element concen-trations within the individual eruption units. Major andtrace element variations, rare earth element (REE) con-centrations and Pb isotope ratios (analyzed on bothpicked separates and in-situ LA-ICP-MS on individualshards spanning a range in SiO2) demonstrated that thecompositional diversity observed on the scale of in-dividual ash shards in the tephra layers was the result ofup to 40% fractional crystallization. These “marine” co-ignimbrite tephras preserved the variation in chemicalheterogeneity present in the silicic magma chambersthat has so far not been evident from bulk geochemi-cal analyses of the welded “continental” ignimbrites(Ukstins Peate et al., 2003, 2005).

This study expands on these preliminary results(Ukstins Peate et al., 2003) and concentrates on char-acterizing the full compositional variations found withinthese tephras. To accomplish this, we have expanded ourstudy to include all four tephra layers from multiplecores. We have generated a detailed dataset of N1300major element electron microprobe analyses on over

Fig. 1. Composite volcanic stratigraphy and paleomagnetic data for on-land Yemen flood volcanism (Ukstins Peate et al., 2005; Riisager et al.,2005). Ages are all 40Ar/39Ar dates from: (1) Baker et al. (1996a),recalculated to the Fish Canyon sanidine standard age of 28.02 Maafter Renne et al. (1998); (2) Ukstins et al. (2002); and (3) Riisageret al. (2005). Correlated equivalent Afro-Arabian ignimbrite units tothe four Indian Ocean tephra layers are annotated. Inset: map of Afro-Arabia showing the location of Sana'a, Yemen, and the distribution ofAfro-Arabian flood volcanics.

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1050 shards, coupled with !400 in-situ laser ablationinductively-couple plasma mass spectrometry (LA-ICP-MS) trace element analyses on shards to capture the fullcompositional spectrum from each ash layer. Togetherwith a few additional Pb and Nd isotope analyses, wewill use these data to revise stratigraphic correlations toon-land units and to discuss the petrogenetic evolutionof these eruptions.

3. Analytical methods

Tephra layers were sampled from two ODP Leg 115drill sites. Hole 711A, deposited below the carbonatecompensation depth, contained only the lower-most anduppermost tephras as visually identifiable layers (Ship-board Scientific Party, 1988). These were the two teph-ras used in our previous study, and we have analysedadditional material from them. Hole 709B, depositedabove the carbonate compensation depth, contained allfour tephras as distinct ash horizons, and we have ana-lysed shards from each of these four layers. The upper-and lower-most tephra are equivalent to 5W and 4W incore 711A, and are also referred to here as 5W and 4W(drill core sample locations are differentiated in the on-line data set Appendix A). The two intermediate tephrasare labeled based on their core sample locations as1W63 (tephra above 5W) and 2W43 (tephra below 4W).ODP sample notation for the location of sample materialsplits from Leg 115 hole 709B are (from top to bottom):4W — 26!1W 9 to 10 cm; 1W63 (one tephra below4W) — 26!1W 63 to 64 cm; 2W43 (one tephra above5W) — 26!2W 43 to 44 cm; and 5W — 26!2W 83 to84 cm. ODP sample notation for the location of samplematerials from hole 711A is found in Ukstins Peate et al.(2003).

3.1. Major element data

Bulk tephra samples were composed of three to fourgrams of ash shards within a siliceous or calcareousooze. Samples were repeatedly rinsed in de-ionizedwater and the liquid–matrix mixture was decanted untilthe water was clear (Ukstins Peate et al., 2003). Cleanshard separates were handpicked and mounted in epoxyfor in-situ analyses. Major element data on the differentshard populations from each of the six samples weremeasured by electron microprobe on a representativeselection of different morphologies (N1000 shards intotal). Most (N99.9%) shards are optically homoge-neous in color. Larger shards were analyzed 2 or 3 timesto assess compositional variations within the shards, andwith the exception of a few heterogeneous shards, most

had SiO2 variations of less than 1 wt.%. Electron mi-croprobe analyses were conducted at the University ofCopenhagen, Denmark, using a JEOL-8000 Superp-robe, following the methodology of Kent et al. (2002a).Analysis totals ranged from 101.5 to 90.2 wt.%, withmost silicic shards (N90%) ranging from 92 to 96 wt.%.Mafic shards had higher total values than silicic shardsand ranged from 96 to 101.5 wt.%, possibly reflectingeither higher primary water contents or secondaryhydration in the more silicic shards. All analyses werere-normalized to 100 wt.% total. The concentrations ofmajor element oxides were calculated by reference to arange of oxide and mineral standards.

3.2. Trace element data

A subset of about 400 shards spanning the full rangein major element compositional variations observed inall tephra samples were selected for additional traceelement analysis by LA-ICP-MS (using the isotopes:43Ca, 26Mg, 47Ti, 45Sc, 51V, 85Rb, 88Sr, 89Y, 90Zr, 93Nb,133Cs, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu,157Gd, 163Dy, 166Er, 172Yb, 178Hf, 181Ta, 208Pb, 232Th,and 238U). Trace element analyses were carried out inthe W. M. Keck Collaboratory for Plasma Spectrometryat Oregon State University using a NewWave DUV193 !m ArF Excimer laser and VG PQ ExCell Quad-rupole ICP-MS following the methods of Kent et al.(2002b). USGS glass standards BHVO-2G, ATHO-G,TI-G, and NIST SRM 612, were analyzed to monitoraccuracy and precision (see on-line Appendix A). Al-though dependant upon elemental abundances in indi-vidual glass samples, external errors calculated frommultiple analyses within individual bracketing runswere typically "5% for most elements (Sc, Ti, V, Cr,Mn, Co, Ni, Rb, Sr, Y, Zr, Nb, Ba, LREE, Er);"10% forDy, Gd, Yb, Hf, Ta, Pb; and"15% for U and Th (at 2!).

3.3. Pb and Nd isotope analyses

Bulk samples (up to!100 mg) of picked glass shardsrepresenting the four main morphologies were analyzedfor Pb isotopes (using a 207Pb–204Pb double-spike:Baker et al., 2004) and Nd isotopes at the University ofCopenhagen following the methods detailed in UkstinsPeate et al. (2003). Shard separates of clear pumice,clear shards, dark pumice and brown shards were sep-arated from 5W and 4W, and clear shards were pickedfrom 1W63 and 2W43. Due to the limited sample sizeavailable for all morphologies besides the dominantclear shards, not all separates contained enough Pb orNd to allow successful isotopic analysis.

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4. Results

Morphologic and geochemical variations of ashshards can be grouped into four distinct shard types(see photos in Table 1): i) clear tricuspate shards andbubble wall fragments (with rare whole hollow glassspheres), which represent the dominant shard type andmorphology, composing N85 modal % of the tephras; ii)pumice, which varies in color from clear to grey andtranslucent light brown, approximately 10 modal %abundance with brown pumice only observed in tephra5W; iii) translucent dark shards, usually with conchoidalfracture surfaces (!3 modal %); and iv) dark opaquebotryoidal ash grains with vitreous luster and a generallyvesicular interior (b2 modal % and only observed in4W). All four morphologies were observed in the up-

permost tephra from both cores (ash layer 4W),morphologies i–iii were observed in the lower-mosttephra from both cores (ash layer 5W), and the two in-termediate tephras contained only clear bubble walls,tricuspate shards and rare white to grey pumiceous ash. Inaddition, tephras 5Wand 4W contain banded shards withclear brown to transparent bands from sub-millimeter tomillimeter-scale variation (Fig. 4A, Table 1).

Vertical systematic changes in shard population with-in individual tephra layers were not addressed in thisstudy. The type iv dark opaque ash particles are dif-ferentiated here from other brown shards (type iii), asthey are now recognized to represent a compositional-ly distinct and geochemically significant component.Shards range in size from approximately 0.2 to 0.5mm indiameter, with the clear cuspate and pumiceous shards

Table 1Shard morphologies and occurrence within each tephra layer

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representing the largest sizes (some flat planar bubblewalls can be up to 0.75 mm in length) and opaque ash thesmallest. The size of the largest flat planar shards isunusual, considering that they represent distal depositsthousands of kilometers from source. Rare euhedral cli-nopyroxene, plagioclase and quartz crystals compose#0.05% of 5W and 4W, and one clinopyroxene wasobserved in 1W63 (Ukstins Peate et al., 2003; Touchardet al., 2003a).

4.1. Major elements

Major element data show that all the tephra layers areextremely heterogeneous in composition, with SiO2

concentrations ranging from 58 to 77 wt.% (5W), 63 to79wt.% (2W43), 66 to 78wt.% (1W63) and 43 to 74wt.%(4W) (Fig. 2; Table 2). Tephras 5W, 1W63 and 4W alldisplay coherent variations over the observed composi-tional ranges, while tephra 2W43 is dominated by com-positions from 74.7 to 76 wt.% SiO2 with just a few shardsmaking up the rest of the spread. Major elements in all

tephras (with the exception of Na) display well-definedcurvilinear trends with systematic variations over the ob-served silica concentration (Fig. 2), and tephra layers withthe largest compositional spans (5W and 4W) display thetightest and most coherent trends. Potassium is positivelycorrelated with increasing silica, and Fe,Mg, Ca and Ti arenegatively correlated with silica enrichment. Aluminumshows a negative correlation with silica in tephra 5W,whereas it displays a systematic increase in the maficshards of 4W, with an apex in concentration and thendecrease in the intermediate to rhyolitic shards (Fig. 2A).When compared with tephra 5W, tephra 4W silicic shardsare systematically offset to lower concentrations of Fe,Mg,Ca and Ti, and higher concentrations of Al, Fe!, Na and Kfor a given silica concentration. The two intermediatetephra layers overlap, or show slightly higher concentra-tions than the field for 5W for all major elements exceptNaand K (see alkali mobility discussion below).

All four tephra layers from core 709B displaysystematically lower alkali contents for a given silicaconcentration than the analyzed shards from core 711A

Fig. 2. Major element variations within individual ash shards from the Indian Ocean tephra layers (from oldest to youngest) 5W (blue circles), 2W43(green diamonds), 1W63 (orange triangles) and 4W (red squares). Data points for each tephra represent individual shard analyses by electronmicroprobe and show the full range of compositional variations found within each ash layer. For ash shards with multiple microprobe analyses pershard, only the averaged analysis for that shard was plotted. This plot does not include the extreme banded shards from 4W and 5W.

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(Fig. 3A, Table 2). For the equivalent tephra horizons5W and 4W, this represents !20% loss of alkalielements (Na2O and K2O) in the shards from core709B. However, highly fluid-mobile trace elementssuch as U form tight linear trends within each tephra(Fig. 3A), as discussed in more detail below, andindicate that other ‘mobile’ elements have not beenaffected by secondary alkali mobility (Zielinski, 1982).Sr and Ba also display coherent trends, but elementconcentrations have been affected by fractional crystal-lization and do not display a tight linear increase overthe full range of SiO2. These samples are of the sametephra layers, with the same major element chemicalcompositions, and were analyzed using identicalanalytical conditions during the same analytical run,therefore Na-mobility during microprobe analysis doesnot appear to be responsible for these variations. Theonly differentiating factor to explain the variation inanalyses between the same tephras found in differentcores is the depositional environment, because Site 711was near or below the carbonate compensation depth(CCD) for most of its depositional history, while Site709 was located above the CCD during the Oligocene(Shipboard Scientific Party, 1988). It appears that thepreservation of primary alkali element concentrations insamples from core 711A is more robust than in core709B, perhaps reflecting variable alkali mobilizationdue to the differing depositional environments, or lo-calized variations in early post-emplacement ion ex-change (Fisher and Schmincke, 1984). Because of this,we determine that while trace element variations appearto be robust for tephras in core 709B (as represented byU trends for example), major element trends are not, dueto sum total effects, and our discussion of major elementvariations in 5W and 4W will be based on data acquiredfrom tephras found in core 711A.

With additional analyses, the lower-most and upper-most ash layers (5W and 4W) display a significantly

larger compositional range than previous observed, andthis is largely due to the twenty-fold larger sample setused for this study (1050 shards versus !50 shards). Inaddition, we purposefully selected a greater proportionof the less-common morphologies (clear to grey pumice,dark pumice, dark shards and opaque shards, as com-pared to the dominant clear shards) than are preserved intrue modal proportions for each tephra layer in order tobroaden the spectrum of analyzed shard types.

4.1.1. Tephra 5WOn a total alkali-silica diagram (Le Bas et al., 1986),

the bulk of the ash shards from the lower-most tephra5W display a continuous, arcuate compositional rangefrom basaltic trachyandesite, trachyandesite and tra-chyte to rhyolite (SiO2: 58.2 to 77.3 wt.% in N400analyses), representing a classic alkali rhyolite fraction-ation trend with increasing SiO2. The average SiO2 ofthe top 5 wt.% (n=114 shards) is 74.8, and the averageSiO2 of the bottom 5 wt.% (n=57 shards) is 60.7. Inaddition, a single banded shard from 5W (Fig. 4A)displays close to the full compositional spectrumobserved within the entire eruption (SiO2: 59.2 to75.3 wt.%, Fig. 4B), preserving !85% of the majorelement chemical variation on the scale of less than amillimeter. This clearly illustrates that the observedcompositional variations within this ash layer representa single eruptive event, irrespective of the mechanism ofdeposition of the tephra layers.

4.1.2. Tephras 2W43 and 1W632W43, the second tephra layer, has a range of SiO2

from 63.3 to 79.3 wt.%. In 2W43, the average SiO2 ofthe top 5 wt.% (n=37 shards) is 75.7 wt.%. 1W63, thethird tephra layer, ranges from 65.9 to 78.4 wt.%. In1W63, the average SiO2 of the top 5 wt.% (n=28shards) is 75.6 wt.%. Of the four tephra layers, 2W43has the most restricted compositional variations.

Table 2Pb double-spike and Nd isotope analyses

Sample 4W light 4W pumice 4W clear dk 4W opaque 2W43 cs 1W63 cs 5W light 5W pumice 5W dk pum 5W clear dk143Nd/144Ndm 0.512860 0.512809 0.512782 0.512891 0.512843 0.512871143Nd/144Ndi 0.512841 0.512790 0.512763 0.512872 0.512824 0.5128522! 0.000012 0.000022 0.000029 0.000011 0.000011 0.000012!Nd 4.70 3.69 3.18 5.29 4.35 4.90± 0.24 0.42 0.56 0.22 0.22 0.24206Pb/204Pb 18.7749 18.7590 18.7564 18.6949 18.7322 19.1555 19.2569 19.1998 18.9997207Pb/204Pb 15.5941 15.5936 15.5969 15.6104 15.6004 15.5906 15.5963 15.5942 15.5943208Pb/204Pb 38.7178 38.7115 38.7000 38.8260 38.6468 38.5817 38.5180 38.4949 38.5942207Pb/206Pb 0.83171 0.83126 0.83155 0.83501 0.83281 0.81390 0.80991 0.81220 0.82077208Pb/206Pb 2.0650 2.0636 2.0633 2.0768 2.0631 2.0141 2.0002 2.0049 2.0313

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Unlike the lower-most and uppermost tephra samples,the core material containing these intermediate ash layerswas not abundant in shards, but the restricted shardmorphology (clear shards and pumice only) and thepredominance of shards in the 74 to 76 wt.% range sug-gest that the analyzed shards are representative of most ofthe chemical variations within these two eruptive units.

4.1.3. Tephra 4WThe uppermost tephra 4W contains shards that range

in composition from picro-basalt, basanite and basalt torhyolite (SiO2: 42.8 to 73.7 wt.%), with a compositionalgap between 60 and 66 wt.% SiO2. Shards from thistephra have been divided into three groups based on

compositional range and breaks in silica content (usinganalyses from core 711A): mafic (n=85, SiO2 from 43to 50, average SiO2: 47.5 wt.%); intermediate (n=14,SiO2 from 52 to 60 and average SiO2: 56.6 wt.%; andfelsic (n=373, SiO2 from 66 to 74 wt.%, average SiO2:69.2 wt.%).

4.2. Trace elements

For all tephra layers Rb, Y, Nb, Cs, Ta, Th, Pb, U andthe REE (rare earth elements) behave as incompatibleelements, increasing with SiO2. Sc and Vare compatibleand decrease with increasing SiO2. Zr is dominantlyincompatible, but in 5W begins to decrease in abun-dance above 76.7 wt.% SiO2, representing the onset ofcompatible behavior due to zircon fractionation. Lightrare earth elements (LREE) are incompatible in all

Fig. 3. Selected major and trace element variations in ash layers 5Wand 4W from ODP Leg 115 cores 711A and 709B, which illustrate thedifferences in alkali mobility in these two core samples. A. SiO2 versusNa2O shows clear Na loss (illustrated by purple field) in both 5W and4W shards from core 709B for a given SiO2 concentration, especiallyat higher SiO2 concentrations. Alkali loss and re-normalization to100% has shifted shard analyses to higher silica contents. B. Nb versusU, an immobile and highly fluid-mobile element, respectively, showtight linear trends for analyses from both tephras in both cores,illustrating that trace elements such as U probably have not beenaffected by secondary mobilization.

Fig. 4. A. Photomicrograph in plain light of a banded shard from tephra5W. B. SiO2 versus TiO2 with the full compositional variation of allshards from 5W shown as blue circles, and compositions measured inthe pictured banded shard shown as green circles. The compositionalvariations observed in the banded shard represent approximately 80%of the variation observed in the entire eruption.

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Fig. 5. SiO2 versus selected LA-ICP-MS trace element data for all tephras. Tephra 5W has continuous compositional variations over 20 wt.% SiO2,4W has two groups of shards, one which ranges frommafic to intermediate compositions (ca. 43 to 60 wt.% SiO2) and a second group of silicic shards(60 to 73 wt.% SiO2). The two intermediate tephras generally fall between the trends of the upper- and lower-most tephras. 2W43 forms acompositional cluster at !76 wt.% SiO2, and 1W63 forms a trend from 66.5 to 78 wt.% SiO2. The lower (main flood basalts) and upper series of theYemen Flood Basalts (YFB) (Fig. 1) are differentiated by variations in concentration of very incompatible to moderately incompatible elements (afterBaker et al., 1996b).

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tephras, but at approximately 75 wt.% SiO2 in 5W and77 wt.% in 1W43, LREE abundances decrease sharply.Likewise, Sr in 4W is incompatible in shard composi-tions up to 68 wt.% SiO2, above which, the onset ofplagioclase fractionation in the rhyolitic magma causesdepletion (Fig. 5H). Ba and Nd (Fig. 5) show a break intrend between the mafic-intermediate shards and thesilicic shards in 4W, with Ba increasing ca. 300% acrossthis break, then decreasing rapidly, and Nd showing ageneral increase ranging from 20 to 60%. In 5W, Bashows a general decrease, but not as abruptly as in thesilicic 4W shards, whereas Nd shows much more scatterfor any given SiO2, but generally increases to 70 wt.%SiO2, and decreases at higher silica contents. Whencompared with tephra 5W, 4W has higher incompatibleelement concentrations of Rb, Zr, Nb, Cs, La, Ce, Pr, SmHf and Ta, and lower concentration of U.

The complex relationships among 5W and the twogroups from 4W are also apparent in other traceelements, such as Sr (Fig. 6D) and V (not shown).Trace element data variations indicate that while tephra5W shows coherent variations in trace element con-

centrations, 4W is more complex and displays two dis-parate compositional groups representing the mafic+intermediate and silicic compositions, which containinternally coherent trends while remaining quite distinctfrom each-other. The two interlayered tephras fall be-tween 5W and 4W compositional arrays. 2W43 forms acluster partly overlapping the highest concentration endof 5W, while 1W63 is intermediate between 5Wand 4Wwith a compositional range equivalent to the higherconcentration group from 4W. Two shards from 2W43plot with 5W. These may represent rare individual oldershards that were re-entrained during a younger eruption,as discussed by Ukstins Peate et al. (2003). There is nosedimentologic evidence to suggest that this is the resultof bioturbation or other process that affected the tephralayers post-deposition, and it is because each tephralayer is compositionally distinct enough that it is pos-sible to use in-situ analyses to identify these rare ac-cidental shards.

We use Nb contents as a fractionation index to assessthe behavior of other incompatible trace elements suchas U, Cs, Eu, and Sr (Fig. 6).

Fig. 6. Nb versus selected LA-ICP-MS trace element data for all tephras. Continuous trends are observed in 5W and 1W63, whereas 4W showsdiscrete differences in trends between the mafic-intermediate shards and the silicic shards. Arrows (panel A, B and D) illustrate changes in slope withincreasing Nb concentration in the two shard compositional groups in 4W, and shaded fields (panel C) show the two distinct compositional groups inNb vs. Cs concentration.

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U: 5W shows a tight positive linear correlation ofincreasing U with increasing Nb (Fig. 6A). Since U isusually a fluid-mobile element, this suggests that thetephra layer has not been significantly affected bysecondary alteration. 4W displays a break in the trend ofincreasing U, with a slight change in slope at ap-proximately 80 ppm Nb.

Cs: Both 5W and 4W have tight positive trends ofincreasing Nb with increasing Cs (Fig. 6C), and 4Wdisplays two compositional groups. The high Nb–Csconcentration group is offset below, and parallel to 5W,whereas the low-concentration group from 4W con-tinues the positive slope of the trend from the lowestconcentrations of the 5W array. These group breaks alsoreflect the groups created by the subtle change in slopeobservable in Nb vs. U.

Eu: Tephra 5W has a well-defined negative trend ofdecreasing Eu at higher Nb concentrations (Fig. 6B).4W is complex, forming two distinct groups in Nbversus Eu space: one group, with low Nb and Euconcentrations, has increasing Nb and Eu up to!60 ppm Nb and 4.5 ppm Eu) at which point Eu levelsoff. The second group of shards from 4W is distinctlydifferent and more enriched in Nb and Eu, initiating at!120 ppm Nb and 8.5 ppm Eu with a negativecorrelation to !180 ppm Nb and 3.5 ppm Eu. 1W63overlaps the range of the higher concentration group ofshards from 4W, and 2W43 forms a cluster, with severalscattered points, in the middle of the higher concentra-tion 4W array.

Sr: 5W shows a well-defined decrease in Sr withincreasing Nb (Fig. 6D). 4W again shows two distinctgroups of shards with increasing Nb: Sr concentrationsfirst increase to a maximum inflection point of ca.1100 ppm and then decrease slightly, suggesting ini-tiation of feldspar fractionation. At slightly higher Nb, thesecond group of shards has significantly lower Sr con-centrations (ca. 100 ppm) that decrease further with in-creasing Nb. 1W63 continues the trend of decreasing Srfrom the higher concentration group of shards from 4W,and 2W43 forms a cluster, with several scattered points, inthe middle of the lower concentration 4W array.

In addition, 5W and 2W43 show a distinct and sharpincrease followed by a decrease in Ba with increasingNb, corresponding to the onset of feldspar fractionationat ca. 72 ppm Nb for 5W and 112 ppm Nb for 1W63,(SiO2 concentrations of 63 and 70 wt.% respectively).Both Sc and V show steep decreases with increasing Nb,suggesting pyroxene fractionation over the full range incomposition. 5W shows decreasing LREE at N70 wt.%SiO2, indicating the fractionation of an accessory phasesuch as apatite.

4.3. Pb and Nd isotopes

On a 207Pb/206Pb versus 208Pb/206Pb ratio plot(Fig. 7A), Afro-Arabian silicic units form two distinctgroups along a linear array, one with high 207Pb/206Pband 208Pb/206Pb, and one with low 207Pb/206Pb and208Pb/206Pb, (Ukstins Peate et al., 2003, 2005).

Fig. 7. A. 207Pb/206Pb versus 208Pb/206Pb isotope variations in picked shard components of all tephras compared to variations observed in Yemensilicic volcanic units. All symbols are labeled in Fig. 7B. Blue fields represent the two Pb isotopic groups that all Yemen silicic units fall into. Whitefield represents in-situ LA-ICP-MS isotopic analyses on individual shards from tephras 5W and 4W (data from Ukstins Peate et al., 2003). Smallerlow Pb isotope fields represent rare, isotopically distinct shards found in both 5Wand 4W, and may represent individual older shards re-entrained inyounger eruptions, as discussed by Ukstins Peate et al. (2003). On-land units are represented by individual symbols, Pb double-spike isotopicanalyses were on picked groundmass chips (Ukstins Peate et al., 2003, 2005), and errors are smaller than the symbols shown. Tephra analyses were onpicked shards of different morphologies (see Table 2). B. 206Pb/204Pb versus !Nd isotope variations in picked shard components of all tephrascompared to variations observed in on-land Yemen silicic volcanic units. Error shown for !Nd is for analysis of groundmass chips for on-land units(see Table 2).

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Differentiating between these two groups has beendiagnostic for constructing volcanostratigraphic correla-tions among terrestrial units in Yemen (Fig. 1: UkstinsPeate et al., 2005). The tephras alternate between thetwo Pb isotope groups, with 5W falling in the lowgroup, 2W43 in the high group, 1W63 in the low groupbut distinct from 5W, and 4W lying at the uppermostextreme of the high group, near 2W43 (Table 2).

Different morphologic groups of ash shards within5W and 4W show heterogeneity in Pb and Nd isotoperatios. In tephra 4W, clear bubble walls, pumice andclear dark shards (where analyzed) display near-identicalPb isotope ratios (207Pb/206Pb: 0.831 to 0.832), whereasopaque dark shards are significantly offset to higher Pbisotope ratios (207Pb/206Pb: 0.835). In tephra 5W, clearshards and dark pumice have Pb isotope ratios that arethe same within error (207Pb/206Pb: 0.810 to 0.812), butdark shards are offset to significantly higher Pb isotoperatios (207Pb/206Pb: 0.821).

The main tephra morphologies analyzed from 5Wand 4W (clear shards) overlap the previous bulk shardPb double-spike analyses from Ukstins Peate et al.(2003), and are identical to in-situ LA-ICP-MS analysesof clear shards which span a wide range in SiO2 (5W 70to 78 wt.% SiO2, 4W 70 to 74 wt.% SiO2; Ukstins Peateet al., 2003), illustrating that the dominant silicic com-ponent within each tephra (represented by clear shards)is isotopically homogeneous (Fig. 7A).

However, Nd isotopes illustrate that the mafic end ofthe compositional range within each tephra is isotopi-cally distinct (Fig. 7B). 4W clear and opaque ash com-ponents have significantly different Nd isotopes (!Nd4W pumice: 4.7, opaque ash: 3.7; Table 2), as do 5Wclear and dark shards (!Nd 5W pumice: 4.4, clear darkshards: 4.9), further supporting the observation that theleast-evolved component in both 5W and 4Wappears tobe isotopically distinct from the bulk of the tephra.Touchard et al. (2003a) analyzed 20 mg of bulk ashseparates from each tephra layer for Nd isotopes, andtheir results generally agree with our analyses for theclear shard component, except for tephra 4W, wheretheir bulk ash data (143Nd/144Nd: 0.512820, !Nd: 3.8)appears be a mixture of the high values we found in thepumice and low values found in the dark component.This is not surprising given the range in isotopic com-position found within the different components and thedifficulty in analyzing a bulk separate of such material.

5. Discussion

The following sections focus on the petrogenesis oftephras 5W and 4W, because they preserve the largest

and most detailed compositional record. Afro-Arabiantephras are unique examples of extreme heterogeneitywhen viewed in terms of eruptive volume, as comparedto other large silicic systems of equivalent size, such asthe ‘monotonous intermediates’ that are very homoge-neous or weakly zoned, as well as remarkably crystal-rich (30–45%; Hildreth, 1981; Whitney and Stormer,1985; de Silva, 1989; de Silva and Wolff, 1995;Bachmann and Bergantz, 2004; Mason et al., 2004).The largest chemical variations observed within indi-vidual eruption units have been documented from: i) theRanier Mesa and Ammonia Tanks Tuffs of the TimberMountain group, Nevada, USA (1200 and 900 km3, andSiO2: 55 to 76 wt.% (Huysken et al., 1994) and 57 to78 wt.% (Mills et al., 1997), respectively), and ii) the!280 km3 Rattlesnake Tuff of eastern Oregon, USA(SiO2: 47.3 to 77.7 wt.% (Streck and Grunder 1999)).

5.1. Revised tephra correlations

Earlier correlations of Indian Ocean tephra layersrelied on major and trace element data and isotopicvariations of the upper-and lower-most tephra layersonly (Ukstins Peate et al., 2003), which were linked tothe Afro-Arabian Jabal Kura'a and Sam Ignimbrites,respectively (Ukstins Peate et al., 2005). New geochem-ical data presented here for the two intermediate tephralayers, and new magnetostratigraphic data for both theon-land Afro-Arabian volcanostratigraphic record andIndian Ocean cores (Touchard et al., 2003a; Riisageret al., 2005) require revised correlations. Afro-Arabiansilicic eruptions generally fall into one of two groupswith distinctive Pb isotope ratios (Figs. 1 and 7A). Thedominant clear shard components of the four IndianOcean tephra layers display Pb isotopes that alternatebetween these two groups: the ‘low’ group (5W and1W83), and the ‘high’ group (2W43 and 4W, Figs. 1 and7A, Table 2). The diagnostic Pb isotope ratios andstratigraphic continuity with continental units that fitthis sequence is shown in Fig. 1. These revisedcorrelations are stratigraphically, geochronologically,geochemically and magnetostratigraphically consistentbetween distal tephra layers and on-land units (Fig. 1).

i. 5W: The correlation of tephra 5W to the JabalKura'a Ignimbrite is confirmed by the new data.

ii. 2W43: Tephra 2W43 is correlated here with theeruption that emplaced the Green Tuff and SamLower Ignimbrite (representing a genetically re-lated and geochemically similar pair of fall andignimbrite from the same eruption). However, aprimary distal tephra is more likely to be derived

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from a co-ignimbrite plume associated with theSam Ignimbrite, due to a significantly larger ashdispersal area based on theoretical and observa-tional models (Sparks and Walker, 1977; Moore,1991; Self, 1992; Woods and Self, 1992; Koya-guchi and Tokuno, 1993).

iii. 1W63: Tephra 1W63 is geochemically correlatedto the isotopically distinctive upper cap of the SamIgnimbrite or overlying Sana'a Ignimbrite, whichpossess identical isotopic compositions and can-not be differentiated chemically (Ukstins Peateet al., 2005). However, based on the stratigraphicposition of these tephras in the ODP cores, it ismore likely that 1W63 is correlated to the Sana'aignimbrite for several reasons (Table 1, Fig. 8). Inholes 711A and 709B, the depositional intervalbetween the peak magnetic susceptibility signalsfrom tephras 1W63 and 4W are more closelylocated (ca. 40 cm) than between 1W63 and theunderlying 2W43 (!75 cm: Shipboard ScientificParty, 1988; Touchard and Rochette, 2004). Basedon average sedimentation rates (6–8 m/Ma;Shipboard Scientific Party, 1988) this implies

time intervals between tephras of ca. 110 and55 ka, respectively, which correlates well with theestablished on-land volcanostratigraphy and geo-chronology (Table 1; Ukstins Peate et al., 2005;Riisager et al., 2005). More significantly, theSana'a Ignimbrite represents the only eruption ofthe Main Silicic Phase emplaced during a periodof reversed polarity in an otherwise long stretch ofnormal polarity (Fig. 1; Ukstins Peate et al.,2005). Paleomagnetic data from cores 711A and709B show that tephra 2W43 was depositedduring a period of reversed polarity (ShipboardScientific Party, 1988; Touchard et al., 2003a) andprovides a conclusive link to the Sana'a Ignim-brite, (Table 1, Fig. 8).

iv. 4W: Tephra 4W is now correlated with IftarAlkalb, a caldera-collapse breccia which overliesthe Sana'a Ignimbrite, but is separated from it byan erosional surface (Ukstins Peate et al., 2005).The ash matrix of the on-land deposit is com-positionally zoned (52 to 72 wt.% SiO2) andcontains fluidal mafic spatter rags of up to 2 m inlength (Ukstins Peate et al., 2005).

Fig. 8. Volume magnetic susceptibility measurements identifying tephras 5W, 2W43, 1W63 and 4W in drill cores from Leg 115 holes 711a and 709b(Touchard et al., 2003a). Paleomagnetic column shows the deposition of tephra 1W63 in a reversed interval (chron C11n.1r), in contrast to the othertephra layers. This correlates with the on-land Sana'a Ignimbrite (Fig. 1), the only unit of the Main Silicic phase to be emplaced in a reversedpaleomagnetic interval (Ukstins Peate et al., 2005; Riisager et al., 2005). Inset map of Afro-Arabia and the Indian Ocean showing the location of ODPLeg 115 drill cores and the distribution of Afro-Arabian flood volcanics.

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5.2. Tephra 4W–Iftar Alkalb: genetic significance ofmafic magmatic component

As addressed by Hildreth (2004), the backgroundflux of mafic recharge into a silicic magmatic systemcreates a compositional gap within an evolved residentmagma, but the distinction between bimodalities intro-duced by new injections and those established by pro-cesses internal to the reservoir need to be distinguished.For example, the mafic shards in 4W–Iftar Alkalb mayrepresent the least-evolved component from which thesilicic tephra was derived, or alternatively, they could begenetically unrelated to the silicic material and representan episode (or episodes) of magma recharge that createda bimodal compositional distribution. One question tobe addressed, therefore, is whether these mafic shardscan be compositionally linked to any of the on-landflood basalts. Baker et al. (1996b) divided the Yemen

flood basalts into two stratigraphically distinct groupsbased on trace element ratios (Fig. 1): (i.) the MainFlood Basalts, which have low ratios of very incompat-ible elements to moderately incompatible elements (e.g.Nb/Zr 0.08–0.13: Fig. 9C) and account for thevolumetrically dominant phase of flood basalt volca-nism, and (ii.) the Upper Series Basalts, restricted involume and areal distribution, but which are intercalatedwith the Main Silicic sequence in the Sana'a area(volcano-stratigraphically unconformably abutting IftarAlkalb, Fig. 1; Baker et al., 1996a,b) and displayenriched incompatible trace elements in comparison tothe main flood basalts (e.g. Nb/Zr 0.13–0.21: Fig. 9C).

Incompatible trace element ratios for the maficshards in 4W–Iftar Alkalb, plot within the field ofUpper Series Basalts (e.g. Nb/Zr and La/Yb: Fig. 9C),and are distinct from the Main Flood Basalts. Isotopicdata for the mafic shards also clearly illustrate that the

Fig. 9. Major and trace element variations in ash shards from 4W separated into mafic, intermediate and silicic components (shown as red, pink andwhite squares, respectively) as compared to the observed on-land Lower and Upper Series flood basalt lavas (shown in blue field where groupedtogether, and separated into dark and light blue fields respectively where moderately to very incompatible trace element concentrations allowdifferentiation) as identified by Baker et al. (1996b). Mafic shards from 4W fall within the field of Yemen flood basalts, and are correlated to theUpper Series based on Nb/Zr ratios (Fig. 1).

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mafic magma is not related to the silicic component, asshown by Pb and Nd isotope ratios, which are identicalwithin error for all other shard types and significantlyoffset from the opaque mafic component (Fig. 7).Therefore, the mafic magma found as a low-volumecomponent in 4W–Iftar Alkalb (b2 wt.%) is not relatedto the volumetrically dominant silicic component, asshown by Pb and Nd isotopic ratios, and is also notrelated to the main phase of flood basalt volcanism, asshown by incompatible trace element ratios. Instead,these mafic magmas are related to the high-Ti UpperSeries Lavas intercalated with the Main Silicic phase ofvolcanism. Substantial compositional heterogeneity isobserved in incompatible trace element ratios for theseUpper Series basaltic lavas within individual volcanos-tratigraphic sequences (Baker et al., 1996b), and thisvariability is comparable to that found within 4W–IftarAlkalb mafic shards.

The onshore lavas have a range of Zr/Y ratios (Bakeret al., 1996b), which indicate variable degrees of partialmelting in the garnet stability field, and demonstrate thatthe mafic lavas are not homogeneous. In addition, theash shard trace element ratios are most similar to asequence of basaltic lava flows most closely associatedin time to the silicic pyroclastic deposits in the field(Sana'a Road section: Baker et al., 1996b). These arefound intercalated with the Main Silicic Sequence in thearea believed to be the caldera source for Iftar Alkalband other ignimbrite eruptions (Ukstins Peate et al.,2005). This reinforces the inferred stratigraphic correla-tions and the link to ignimbrite units, and is not entirelysurprising, as the temporal record of volcanismillustrates that both mafic and felsic eruptions occurredpenecontemporaneously (Fig. 1; Baker et al., 1996a,b;Ukstins Peate et al., 2003; Riisager et al., 2005; UkstinsPeate et al., 2005). It demonstrates that these tephras areclearly linked to the Main Silicic phase of thevolcanostratigraphy (Fig. 1) and reflects that UpperSeries mafic magmas were spatially and temporallyassociated with pre-existing silicic magma chambers.

5.3. Tephra 4W–Iftar Alkalb: origin of intermediateshards

Intermediate shards in 4W–Iftar Alkalb (SiO2: 52.3to 60.4,#5 vol.%) are associated with the mafic shardsin that they extend the mafic trend towards moreevolved compositions and are separated from the silicicshards by a 6 wt.% gap in SiO2. As such, theintermediate compositions may represent: (i) extendedfractional crystallization of the mafic magma, (ii) theearliest, least-evolved component related to the more

silicic compositions, or (iii) mixing or mingling betweengenetically distinct mafic and silicic constituents in themagma chamber.

In general, the trace element data suggest a closerrelationship for the intermediate shards with the maficshards rather than the silicic shards, as the compositionaltrends observed with the mafic shards continue in theintermediate shards (Fig. 6). For example, on Fig. 6Athe intermediate shards fall along the shallow trend of Uversus Nb, which intersects the steeper trend observed inthe silicic shards, and on Fig. 6C, the intermediateshards lie along the more enriched mafic Cs trend,significantly offset from the silicic trend. A plot of Srversus SiO2 (Fig. 9D) illustrates the compositionaldifferences between the mafic and silicic componentswithin 4W–Iftar Alkalb and can be used to assesswhether the intermediate shards represent mafic-silicichybrid mixtures. The intermediate shards form a trendextending from the mafic magma of uniformly high Sr(!950 ppm) with increasing SiO2, in contrast to thesilicic shards with low Sr concentrations (!175 to10 ppm; Fig. 9D). One silicic shard (SiO2: 68 wt.%)extends this mafic to intermediate shard trend further,and one intermediate shard (SiO2: 59 wt.%) plots at lowSr concentrations (519 ppm). The same pattern occursfor Eu, Rb and Ba (Fig. 5). If mafic and silicic end-members mixed to produce intermediate compositions,shards would fall on a linear mixing line between thetwo Sr end-member components. The shaded field(Fig. 9D) represents the compositions that would resultfrom mixing of any observed mafic shard compositionwith any silicic shard composition. The general high Srconcentrations of the intermediate shards, equivalent tothe mafic magma and significantly higher than the silicicshards, suggests that intermediate compositions are notproducts of hybridization between co-existing mafic andsilicic magmas (the single low-Sr shard may be anexception). Instead, intermediate shards represent frac-tionated products of the mafic magmatic component,with calculated F values of up to 57% (based on therange in Nb concentrations).

5.4. Petrogenesis of the silicic tephra shards

The evolved component in Afro-Arabian tephras(herein referred to as ‘silicic’) shows a broad andcontinuous compositional variation that is especiallynotable in 5W–Jabal Kura'a and 4W–Iftar Alkalb,ranging from intermediate to high-silica rhyolite (5W–Jabal Kura'a: 57 to 79 wt.% SiO2; 4W–Iftar Alkalb: 66to 76 wt.% SiO2), and is volumetrically dominated byrhyolitic compositions (N80% by volume). When

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considering petrogenesis, it is important to differentiatebetween generating geochemical variations withincompositionally zoned rhyolites and the origin of theleast-evolved silicic composition found in that compo-sitional suite, since the processes responsible for theirgeneration may differ (Streck and Grunder, in press). Anexample of such divergent processes is partial melting ofmafic underplate to generate the least-evolved siliciccomponent, followed by extreme fractional crystalliza-tion and magma mixing to generate silicic variations, asobserved in the Rattlesnake Tuff (Streck and Grunder,1997), and the Macolod Corridor, Luzon, Philippines(Vogel et al., 2006). It is clear that the least-evolvedcomponents in all four Afro-Arabian tephras havedistinct compositional differences (e.g. Al2O3, Fe2O3,Sr, Nb, Eu, U and Cs: Figs. 2, 5, and 6, Table 2) thatrequire differing petrogenetic conditions.

Two different mechanisms can generate the compo-sitional variations observed within the Afro-Arabiantephras: mixing between two or more end-memberspresent in the magmatic system, or heterogeneitiesdeveloped during protracted periods of fractionalcrystallization, or a combination of both. Magma mixingbetween distinct end-members generates linear compo-sitional variations in major and trace elements, whereasfractional crystallization produces non-linear trends.The following discussion will focus on identifying ifeither of these processes was primarily responsible forthe observed compositional heterogeneities. We are notconsidering assimilation of country rock because of thelack of isotopic variation within the population of silicicshards (Fig. 7 and Table 2).

Major and trace element variations in the siliciccomponent from tephras 5W–Jabal Kura'a and 4W–Iftar Alkalb show trends that appear linear and areconsistent with magma mixing. Elements such as Fe2O3!(total Fe recalculated as Fe2O3) and MgO displaycoherent and well-defined decreasing curvilinear trendswith increasing SiO2 (Fig. 2). Conversely, traceelements such as Nb and U show tight positive linearcorrelations (Fig. 6). Other elements such as Al2O3

show a change in slope with increasing SiO2 (Fig. 2).Trace elements, however, display non-linear trends thatclearly indicate binary mixing cannot produce theobserved variations (Zr, LREE: Fig. 5C).

Isotope compositions of the silicic component arehomogeneous within each tephra regardless of major ortrace element variation. In-situ LA-ICP-MS on clearshards from 5W–Jabal Kura'a spanning a wide range inSiO2 (70 to 78 wt.%, Ukstins Peate et al., 2003)illustrates that the most silicic compositions areisotopically homogeneous with respect to Pb (Ukstins

Peate et al., 2003), and laser ablation analyses agree withour Pb double-spike analysis of picked clear shards anddark pumice (Fig. 7). Additionally, Pb double-spikeanalyses for three of the most silicic morphologiccomponents in 4W are identical within error (Fig. 7,Table 2), and agree with in-situ LA-ICP-MS on shardsfrom 70 to 74 wt.% SiO2.

The continuous, coherent and non-linear trends intrace elements observed in the silicic component in 5W–Jabal Kura'a and 4W–Iftar Alkalb (Figs. 2, 5, and 6) andhomogeneous isotopic compositions are consistent withcrystal fractionation from a least-evolved silicic magmaas a primary process controlling compositional varia-tions within the silicic component of the tephras. Pb andNd isotope ratios for the main shard morphologies in4W–Iftar Alkalb show that the full compositional spec-trum observed in the silicic component is geneticallyrelated, and isotopically distinct from the mafic shards(Table 2, Fig. 7). 5W–Jabal Kura'a is more complex,because although there are continuous and coherentvariations in major and trace elements, Pb and Ndisotope variations indicate that dark shards, representinga low-silica end-member, have distinct isotopic signa-tures and are not, therefore, directly genetically relatedto the more silicic component (Table 2, Fig. 7). This maybe illustrated by some trace element variations in 5W–Jabal Kura'a (Sr, Pb: Figs. 5 and 6), which show morescatter in the least-evolved compositions, but convergewith an increasing degree of fractionation. Awide rangein Nd and Eu concentrations and Nb/Zr ratios for a givenSiO2 are also observed (Fig. 5). The fact that differentshard populations retain distinct Pb and Nd isotoperatios indicate that multiple magmas co-existed withoutsignificant mixing prior to eruption, yet the lack ofdiscrete compositional clusters in major and traceelements suggests that those magma batches were verysimilar, and perhaps only resolvable with fine-scalegeochemical analysis on individual shards.

Least-squares modeling of fractional crystallizationwas carried out for the observed silicic compositionalvariations in 4W and 5W, utilizing phenocryst composi-tions measured in on-land equivalent units Jabal Kura'aand Iftar Alkalb (Ukstins Peate, 2003). The compositionalvariations observed in the silicic component of 4W areconsistent with fractional crystallization of anorthoclase,augite, titano-magnetite and ilmenite (in the proportionsanor: 81%, aug: 12%, mag: 3%, ilm: 4%), as observed inthe on-land phenocryst assemblage. The silicic componentof 5W spans a larger range in composition, and the moresilicic portion of that array is also consistent withcrystallization of the same phases in the same proportions.For the less-evolved portion of the array, modeling

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suggests that plagioclase is the dominant fractionatingphase, along with anorthoclase, more substantial amountsof augite, titano-magnetite and ilmenite (in the proportionsplag: 40%, anor: 21%, aug: 26%, mag: 8%, Ilm: 5%).

Least-squares modeling fractional crystallizationresults agree with observed phenocryst assemblages,and support extreme fractional crystallization to gener-ate the silicic compositional variations observed in thetephras. Nb concentrations can be used to approximatethe extent of crystallization recorded by the ash shards.Assuming Nb is perfectly incompatible, the observedchange in Nb concentrations for ash layers 5W–JabalKura'a, 1W63-Sana'a and 4W–Iftar Alkalb yieldF values of 0.41, 0.56 and 0.5, respectively, for thesilicic shards, indicating that a minimum of 50% to 60%fractional crystallization has occurred to produce therange of evolved compositions observed in these teph-ras. Continuous compositional variations were generat-ed and maintained in a stratified magma chamber,unmixed, and eruption resulted in magma mingling ofthe entire spectrum of magmatic compositions, as evi-dent from rare banded shards containing giant compo-sitional variations (Fig. 5).

The origins of the least-evolved silicic componentsfound in all eruptions cannot be unambiguouslyconstrained, since there is no conclusive evidence todifferentiate between extreme fractional crystallizationof basaltic magma, partial melting of mantle-derivedand newly formed basaltic underplate, or a combinationof those processes. In any case, it is plausible that theultimate heat and material source of the least-evolvedsilicic component was the main flux of flood basaltmagmatism, based on the volumetric requirements togenerate voluminous silicic magmas. The timing rela-tionships between the peak of flood basalt volcanismand the production of the silicic eruptions (!0.4 Ma),and the general similarities in trace elements (such as Sr)and isotope ratios (such as O, Sr, Nd and Pb) betweenthe mafic flood basalts and silicic pyroclastic depositsalso support this (Baker et al., 1996b, 2000; UkstinsPeate et al., 2003; Ukstins Peate et al., 2005).

6. Summary

Compositionally heterogeneous Afro-Arabian tephras,found in distal Indian Ocean ash layers, preserve acomplex and detailed record of magma chamberprocesses not recognized from the equivalent on-landignimbrites and ash deposits.

1. Four distal tephras from the Indian Ocean can becorrelated to individual ignimbrites found in the

Afro-Arabian flood volcanic stratigraphy based onvolcanostratigraphic, geochemical and paleomagnet-ic data.

2. Individual tephra layers have compositional varia-tions of up to 32 wt.% SiO2, from 43 to 74 wt.%,preserving among the largest geochemical composi-tional heterogeneities observed in single eruptiveevents, and not apparent from on-land whole-rockcompositional analyses.

3. Silicic compositional variations were generated byextreme fractional crystallization of a least-silicicend-member (with 57–66% SiO2), and least-squaresmodeling requires a minimum of 60% fractionalcrystallization of the phases anorthosite, augite, mag-netite, ilmenite±plagioclase, as observed in the on-land phenocryst assemblage, to generate the ob-served compositional variation.

4. Banded shards found within individual tephra layers5W and 4W display the full compositional variationof each tephra, clearly demonstrating that the com-positional variations found in each tephra representsa magmatic system.

5. The mafic component found in the uppermost tephra(4W–Iftar Alkalb) is geochemically correlated to theUpper Series basalts, and not the main phase of floodbasalt volcanism. This mafic magma existed contem-poraneously with the silicic system, and was tappedduring the culminating eruption of the Main Silicicphase of Afro-Arabian flood volcanism.

Acknowledgements

We would like to thank Scott Bryan and David Peatefor critiques and discussion about this research, andShan de Silva, Mike McCurry and Scott Bryan forconstructive reviews of the manuscript. IUP's researchwas partly supported by a post-doctoral research fel-lowship from the Danish Lithosphere Centre. TheDanish Lithosphere Centre was funded by the DanishNational Research Council. This paper was written withpartial support from NSF grant 0439888 to IUP. Wethank the Oregon State University Geology Department,COAS microprobe facility, The Beanery, Frank TepleyIII and Mike Rowe for valuable assistance during ourvisits. We thank the Ocean Drilling Program core re-pository for providing sample materials.

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.lithos.2007.08.015.

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